This invention relates generally to telephone communication systems and more particularly to interface circuits for use in telephone communication systems.
As it is known in the art, telephone communication systems generally include a plurality of remote users, or telephone subscribers, and a central office. The central office has a plurality of pairs of terminals, generally referred to as tip/ring terminals, with each pair coupled to a corresponding one of the plurality of subscribers. The central office electrically routes one of the plurality of subscribers (i.e. the "calling" subscriber) to another one of the plurality of subscribers (i.e. the "called" subscriber) so that analog voice and/or digital data information may pass therebetween. Specifically, the central office electrically connects the pair of tip/ring terminals coupled to the "calling" subscriber to the pair of tip/ring terminals coupled to the "called" subscriber.
More particularly, the central office provides a direct current (DC) voltage source, typically having a level of -48 volts, coupled to each of the pairs of central office tip/ring terminals, and thus to the one of the plurality of subscribers coupled thereto. The DC voltage is used to power circuitry within the telephone when analog voice and/or digital data information is transmitted or received. When a subscriber, or more specifically a telephone receiver, is "on hook", an open circuit condition exists and current is prevented from flowing from the DC voltage source through the telephone. When the telephone receiver goes "off hook", for example when the subscriber places a call, a closed circuit condition is provided and a DC current flows from the DC voltage source through the telephone (i.e. the telephone "sinks" a DC current). This DC "sinking" action is sensed at the central office and appropriate action is taken, for example the central office sends such subscriber a "dial tone" signal. The "calling" subscriber then enters the telephone number of a selected, "called" subscriber and in response to such dialing information, the central office "rings" the telephone of the "called" subscriber by providing a 20 Hertz AC "ring" voltage from a "ring" voltage source, superimposed on a DC voltage also provided by such "ring" voltage source, to the "called" subscriber. If the corresponding subscriber is "off hook", then the central office will not apply the 20 Hz AC "ring" voltage to the telephone of the corresponding subscriber and the central office will send a "busy signal" to the "calling" subscriber. If the "called" subscriber is "on hook", then the 20 Hz AC "ring" voltage "rings" the telephone of such subscriber. If the "called" subscriber responds to the "ring" and goes "off hook", a closed circuit condition is provided and the "called" telephone "sinks" DC current and AC current from the "ring" voltage source provided by the central office at the pair of central office tip/ring terminals coupled thereto. The central office senses this "off hook" condition, or the DC current "sinking", occurring in the "called" subscriber, and electrically decouples the "ring" voltage source from the tip/ring terminals and couples the DC voltage source to such subscriber. The central office also couples together the central office tip/ring terminals coupled to the "calling" and "called" subscribers, so that information may pass therebetween.
Often, when the subscribers are located at a relatively long distance away from the central office, for example greater than approximately 200 miles, the telephone communication system uses a digital loop carrier system which provides an interface between the central office and the subscribers. The digital loop carrier system generally includes a pair of switching systems coupled together via a high speed digital transmission line. One of the pair of switching systems is located near the subscribers, and may be referred to as the remote terminal switching system, and the other switching system is located near the central office, and may be referred to as the central office terminal switching system. Each of the switching systems provides time division multiplexing and demultiplexing, such that, depending on whether information is being transmitted from one of the subscribers to the central office or from the central office to one of the subscribers, one of the switching systems provides time division multiplexing while the other provides time division demultiplexing. With such an arrangement, the digital loop carrier system effectively localizes the central office to the town and is transparent to both the central office and subscribers.
More particularly, each one of the plurality of subscribers includes a pair of terminals, also generally referred to as the tip/ring terminals, coupled to the remote terminal switching system through a corresponding one of a plurality of remote terminal (RT) interface circuits. Similarly, each one of the pairs of central office tip/ring terminals is coupled to the central office terminal switching system through a corresponding one of a plurality of central office terminal (COT) interface circuits. In order for the digital loop carrier system to be transparent to the central office and to the subscribers, each of the COT interface circuits must appear to the central office as a telephone would appear (i.e. inter alia have the capability to "sink" a DC current when the corresponding subscriber is "off hook") and each of the RT interface circuits must appear to the corresponding subscriber as a central office (i.e. provide, inter alia, a nominal -48 volt DC source).
In a conventional COT interface circuit, serially disposed between the tip/ring terminals, are: the primary winding of a transformer, a capacitor in parallel with a resistor, and a relay. The secondary winding of the transformer is coupled to the central office terminal switching system through an analog to digital and digital to analog converter, or CODEC. Thus, the analog voice and/or digital data information is coupled between the tip/ring terminals of the COT interface circuit and the CODEC through the transformer by mutual inductive coupling between the primary and secondary windings. The transformer provides isolation to the CODEC so that it is not damaged by the flow of DC current. The relay is opened or closed selectively in response to a hook condition signal generated at the corresponding RT interface circuit and transmitted to the corresponding COT interface circuit through the remote terminal and central office terminal switching systems and the digital transmission line. The hook condition signal indicates to the COT interface circuit whether the corresponding subscriber is "on hook" or "off hook." When the corresponding subscriber is "off hook", the relay is closed, providing a closed circuit condition, to permit DC current "sinking" as referred to above. Here again, the central office senses this "off hook" condition, or the DC current "sinking" action, and appropriate action is taken. When such subscriber is "on hook", the relay is opened, providing the open circuit condition referred to above.
Since, as previously noted, the transformer couples analog voice and/or digital data information between the tip/ring terminals and the CODEC by mutual inductive coupling, it is desirable to operate the transformer in a non-saturated condition. It is also noted that, when the corresponding subscriber is in the "off hook" condition, the relay closes and current from the DC voltage source at the central office passes between the tip/ring terminals of the COT through the primary winding of the transformer and the serially connected resistor to permit the above described DC current "sinking" action. Thus, the DC current will tend to drive the transformer into saturation and thereby reduce its coupling effectiveness. One technique used to reduce the tendency of the transformer toward saturation, is to use a relatively large and expensive transformer, since the size of the transformer is related to the saturating flux density level and hence the DC current handling level of the transformer prior to saturation.
The relay used on a conventional COT interface circuit is generally referred to as the "hook relay", and has an additional resistor and capacitor disposed in series with each other and in parallel with the relay so that when the switch is open, as is the case when the corresponding subscriber is "on hook", information, for example from a computer and modem arrangement, is coupled through the capacitor and resistor. However, the AC transmission loss associated with the information coupled through the capacitor and resistor may cause signal distortion. This relay is generally a relatively large and expensive component.
As it is also known in the art, while the use of a digital loop carrier system eliminates the cost of providing a central office in small, remotely located towns having relatively few subscribers, networks using a digital loop carrier system require inventories of both RT interface circuits and COT interface circuits. To maintain both inventories is relatively expensive. Further, in certain applications, a subscriber may choose to have a private branch exchange (PBX) in place of an existing telephone. With a PBX, it is necessary that the functionality of the corresponding RT and COT interface circuits be reversed. That is, because a PBX requires central office terminal functionality where remote terminal functionality was provided for telephone subscribers, corresponding interface circuits associated with a PBX would require interchanging. Thus, when a change is desired, a service technician is required to travel to the location of the remote terminal interface circuit to make the interchange.